It is known as usual, in the oil industry, to use cracking processes wherein the hydrocarbon molecules of high molecular weight and high boiling point are split into smaller molecules whose boiling point may be in lower temperature ranges, adapted to the desired use.
Presently, the most usual process for this purpose is the so-called fluid catalytic cracking or FCC process. In this type of process, the hydrocarbon charge is vaporized and contacted at high temperature with a cracking catalyst which is maintained suspended in the charge vapors. When the desired molecular weight has been reached by cracking, with a corresponding decrease of the boiling points, the catalyst is separated from the obtained products, stripped, regenerated by combustion of the formed coke and then again contacted with the charge to be cracked.
In a new FCC process, called R2R, two regeneration zones are used wherethrough circulates the used catalyst.
The charges to be cracked are usually injected in the reaction zone at a temperature generally ranging from 80.degree. to 400.degree. C., under a relative pressure of 0.7 to 3.5 bars, the temperature of the regenerated catalyst reaching said zone being of about 600.degree. to 950.degree. C.
For sake of clarity, FIGS. 1 and 2 show the known usual structure of a reactor of tubular type with double regeneration; however, according to the present invention, any convenient reactor, conventional and not necessarily of the "riser" type (tube with ascending flow of the charge and of the catalyst) can be used. Other existing reactors are shaped as tubes with downward flow of the fluids from a line not shown. The catalyst is introduced at the bottom of the charge elevator 1 of the riser type through a line 2, in an amount determined by the opening or the closure of a valve 3. The catalyst particles are then ejected towards the top of the riser by injecting at the bottom of the latter a gaseous fluid supplied from a line not shown; this injection is performed through a fluid distributor. The charge to be cracked is introduced at an upper level through a line, not shown, and vaporized through a suitable device within the dense flow of the catalyst particles.
The top of column or riser 52 opens into an enclosure 1 which, for example, is concentric thereto and wherein are performed, on the one hand, the separation of the cracked charge and, on the other hand the stripping of the used catalyst. The treated charge is separated in a cyclonic system which is housed in enclosure 1, at the top of which is provided a discharge line 50 for the cracked charged, whereas the used catalyst particles are reintroduced at the bottom of enclosure 1, by means of any suitable device such for example as the T-member 51.
The so-stripped particles of used catalyst are discharged from the bottom of enclosure 1 towards a first regenerator 4 through a line 3 provided with a regulating valve. In regenerator 4, the coke deposited on the catalyst particles is burnt with air injected from the bottom of the regenerator through a line 39, via regularly spaced injectors 55. The particles of the treated catalyst and the combustion gas are separated in cyclones 56, wherefrom the combustion gas is discharged through line 5.
The catalyst particles having undergone a first partial regeneration treatment are then transferred to a second stage 21 of the regenerator through the central duct 58.
The bottom part of stage 21 is also fed with air through a line 20 and injectors 54. The particles of regenerated catalyst are laterally discharged towards a buffer enclosure 59 and are recycled through line 2 to the riser feeding system. The combustion gases discharged from the upper part of stage 21 are treated in an external cyclonic system 60 which accordingly is adapted to perfectly withstand the high temperatures resulting from a complete combustion of coke and at the bottom of which the catalyst particles are fed back, through line 61, to stage 4, whereas the combustion gases are discharged through line 22.
This assembly of catalytic cracking unit, equiped with a two-stage regenerator with upward flow, has the following advantages:
double regeneration of the catalyst, whereby a complete combustion of coke is achieved without alteration of the catalytic properties, PA1 no temperature limitation in the second regenerator, whereby the catalyst can reach the temperature required for vaporizing the charge, mainly when the latter is a heavy charge, PA1 improvement of the thermal stability and of the catalyst resistance to metals.
The hydrocarbons charges convenient as feed for units of the type shown may contain hydrocarbons having boiling ranges from 200.degree. to 550.degree. C. or more and a density from 10.degree. to 35.degree. API, but they may also consist of heavy charges containing hydrocarbons whose boiling point may be as high as 750.degree. C. and more and whose density may be from 10.degree. to 35.degree. API.
The catalysts adapted for use in the above-described devices comprise the cracking catalysts of the crystalline alumino-silicate type, certain types of silica-alumina, of silica-magnesia, of silica-zirconium, all having relatively high cracking activities.
Crystalline alumino-silicates may be natural or prepared by synthesis according to techniques well-known in the art. They may be selected from synthetic zeolites or clays such as faujasite, certain mordenites, montmorillonite, bridged clays, alumino-phosphates or similar substances.
In certain FCC units, a recovery turbine is used in the fumes circuit. But it must be known that the use of these different techniques, which show the interest of recovering power still available at the regenerator output, is accompanied with many difficulties associated with the insertion of the recovery turbine in the unit. FIG. 1 illustrates the use of a turbine in a unit of FCC process of R2R type. In this unit, the fumes issued from the first regenerator wherethrough passes the catalyst is supplied to a turbine.